It doesn't seem like this project has been maintained in quite a few years and while it's referenced in the Griddler GitHub, the online docs from ThoughBot does not mention Mailgun:
Griddler is a Rails engine that provides an endpoint for SendGrid, Cloudmailin, Postmark or Mandrill and hands off the email to your application.
Would you recommend against building on this adapter?
It doesn't seem like this project has been maintained in quite a few years and while it's referenced in the Griddler GitHub, the online docs from ThoughBot does not mention Mailgun:
Would you recommend against building on this adapter?